How do I let go?

My boyfriend revealed to me a couple months into our relationship that he had a romantic encounter with an adult when he was still in high school (he was 18 but she was a teacher :) nice right?) she was either just divorced or still married and she had always had relations with students!! Before they started anything romantic she accidentally sent him a picture of her nude boobs, what a common accident to send to your student. I disliked her because she was very vulgar with personal life when she taught me (several years after my boyfriend was in high school, we have a 4 year age difference so I didnt know him when he was in high school and therefore didnt know about the romantic encounter nor did I know her) and she also was the first person to ever send me to internal over a poster that I made in her supply room (was always locked and she gave me full access to) I still hate her to this day and have second hand hate for my boyfriend when ever I think of him being with her and I want to move past it. Help